This invention relates to electric current-limiting fuses for elevated circuit voltages, in particular for circuit voltages from about 5 kv to voltages in excess of 30 kv.
This invention is predicated on the application on the method of assembling high-voltage fuses disclosed and claimed in the U.S. Pat. No. 3,848,214 to Erwin Salzer, 11/12/74 for METHOD OF ASSEMBLING ELECTRIC HIGH-VOLTAGE FUSES AND SUBASSEMBLY THEREFOR, assigned to the same assignee as the present invention.
This invention is also an outgrowth of the structure disclosed and claimed in my U.S. Pat. No. 3,810,062, 05/07/74 for HIGH VOLTAGE FUSE HAVING FULL RANGE CLEARING ABILITY, assigned to the same assignee as the present invention.
Before considering the relation of the present invention to the respective subject-matter of the two above patents, some brief general historical remarks relating to the development of high-voltage circuit interrupting devices seem to be in order.
One of the earliest high-voltage circuit interrupting devices was the plain oil circuit breaker. In such circuit breakers an electric arc is drawn by separation of a pair of contacts under oil. The ensuring arc breaks down the oil, thus forming a gas bubble or arc bubble surrounding the arc. The high thermal conductivity of the hydrogen in that gas bubble results in relatively rapid cooling of the arc and consequent arc extinction. Thus in the plain oil circuit breaker arc extinction as such is essentially a static rather than a dynamic process involving rapid flows of de-ionizing media. As time went on the requirements in regard to interruption of high-voltage circuits became more onerous and then the above essentially static mechanism of arc-extinction was not adequate any longer. Because of this inadequacy many novel families of circuit breakers evolved, all having the common feature that arc-extinction is effected by dynamic processes involving the rapid flow of de-ionizing media. The explosion pot circuit breaker, the oil impulse circuit breaker, the air blast circuit breaker and modern SF.sub.6 circuit breakers are predicated upon dynamic processes, involving the rapid flow of de-ionizing media.
The current-limiting fuse as it has been known to-date is essentially a static circuit interrupting device, predicated on the high heat of fusion of quartz sand rather than on establishing closely controlled jets of arc-extinguishing gases. It is true that current-limited fuses were sometimes provided with structures that evolve gases under the heat of the electric arcs, but such structures were provided mainly to make it possible for current-limiting fuses to interrupt extremely small overload currents, e.g. overload currents in the range of the 1 hour fusing current, or slightly smaller, or slightly larger, overlaod currents.
In conventional designs of high-voltage fuses the fusible element is wound helically around a mandrel, or supporting core, of a material that evolves arc-quenching gases under the action of electric arcs. This arrangement results generally in evolution of too much arc-quenching gas when such fuses are called upon to interrupt major fault currents. Generation of high pressures caused by generation of very large quantities of arc-quenching gases is conducive to bursting of the casings of the fuse, or to excessive requirements in regard to the bursting strength thereof. These drawbacks can be avoided by using composite mandrels for supporting the fusible elements, portions of which mandrels being made of gas-evolving materials, while other portions thereof are made of non-gas-evolving materials. The cost-effectiveness of this type of fuses is not very satisfactory. My above referred-to U.S. Pat. No. 3,810,062 as well as my U.S. Pat. No. 3,864,655 teach the use of beads of a gas-evolving material, combined with overlays of low fusing point materials on the fusible element (so-called M-effect causing materials) as means for effectively interrupting overload currents of very small magnitude, thus arriving at a novel type of high-voltage fuses having full range clearing ability. I have discovered that high-voltage fuses as shown in my U.S. Pat. No. 3,810,062 not only enhance their small overload performance, but have also the tendency to enhance their major fault current interrupting performance, provided the various paramters which go into their design are correctly selected to achieve this end.
Non-gas-evolving mandrels for supporting fusible elements or non-gas-evolving fusible element supporting cores are used in high-voltage fuses wherein the interrupting capacity is limited at the lower end of the current interrupting range. In such fuses the current interrupting process is essentially static in the sense that their arc-extinction is essentially predicated on the heat absorbing action of granular SiO.sub.2 rather than the action of closely controlled fluid blasts.
Considering now fuses having mandrels for supporting their fusible elements that are made entirely of a gas-evolving material, as mentioned above, in such fuses the amount of gas evolved under major fault current conditions tends to be excessive on account of the fact that gas-evolution increases as the current intensity increases, and also on account of the fact that the number of points at which gas is evolved from such mandrels or cores is equal to the number of points at which the mandrel or core is engaged by the fusible element, or elements. In such fuses the effectiveness under major fault current conditions of the large number of gas-evolving points in regard to generation of arc voltage is relatively small, because the arc is not restricted to, or fixedly held at, the points of gas generation, but is induced to move away from these points into the arc-quenching filler where the arc is quenched by the filler's heat absorbing action rather than by dynamic fluid jet action.
Since the number and the size of gas-evolving beads in a high-voltage fuse as disclosed in U.S. Pat. No. 3,810,062 can be selected as deemed necessary, or desirable, the danger of excessive gas evolution at major fault currents can be effectively avoided in such a fuse. If a break or an arc is formed in a bead of a high-voltage fuse as disclosed in U.S. Pat. No. 3,810,062, the arc cannot be displaced out of the bore in the bead in which it is trapped, but is held captive in the bore, and while held captive inside the latter, subject to the action of highly effective blasts of arc extinguishing gas. These blasts of gas originate inside the bore of each gas-evolving bead and extend axially outwardly in opposite directions. If the cross-section of the bores in the beads is sufficiently small to preclude, or to minimize, entry of particles of pulverulent arc-extinguishing filler or quartz sand into the bores, no fulgurites can form inside of the bores. Hence each gas-evolving bead and its bore forms an interruption of the substantially helical fulgurite which replaces the fusible element following blowing of the fuse. These interruptions preclude the flow of current through the fulgurite following extinction of the arc discharge.
The fuse disclosed in U.S. Pat. No. 3,810,062 has no mandrel or supporting core for the fusible element, the supporting function of the fusible element being solely achieved by the pulverulent arc-quenching filler as such and the gas-evolving action being achieved by the gas-evolving beads.
The above is an outgrowth of the structure disclosed in U.S. Pat. No. 3,810,062 and of its actual performance.
The present invention is based on the above analysis, and is particularly concerned with an optimal positioning of gas-evolving beads for the purpose of optimizing the performance of the fuse under major fault current conditions rather than under overload conditions.